The present invention relates to an active filter circuit and more particularly to an active filter circuit suitable to be incorporated into a semiconductor integrated circuit.
FIG. 1 is a circuit diagram illustrative of a conventional active filter circuit. The conventional active filter circuit has an operational amplifier 60 having two inputs and a single outputs wherein an inverted input and the single output are connected to an output terminal 66 of the conventional active filter circuit, while a non-inverted input is connected through a capacitor 61 to a ground. The non-inverted input of the operational amplifier 60 is also connected through a series connection of two resistances 63 and 64 to an input terminal 65 of the conventional active filter circuit. An intermediate point between the two resistances 63 and 64 is also connected through a capacitor 62 to the output of the operational amplifier 60.
The above active filter acts as a low-pass filter. The first and second resistances 63 and 64 have the same resistance xe2x80x9cRxe2x80x9d. The first capacitor 61 has a first capacitance xe2x80x9cCxe2x80x9d whilst the second capacitor 62 has a second capacitance xe2x80x9c2Cxe2x80x9d which is larger by two times than the first capacitance. A cut-off frequency fcc of the active filter is given by
fcc=1/{square root over ((2xcfx80xc3x972+L CR))}xe2x80x83xe2x80x83(1).
Actually, however, it is difficult to form resistances and capacitances which accurately correspond to the design values on the semiconductor substrate for realizing the semiconductor integrated circuit. If the above conventional active filter circuit of FIG. 1 is formed in the semiconductor integrated circuit, then the resistances and capacitances are likely to be different from the design values, for which reason the cut-off frequency fcc is also likely to be different from the design value. If the design values are R=80 kxcexa9 and C=60 pF, then the design value of the cut-off frequency fcc is found to be fcc=23.4 kHz in accordance with the above equation (1). Actuarially, however, if the actual resistance value is unintentionally increased by 15% up to R=92 kxcexa9 and the actual capacitance is unintentionally increased by 13% up to C=68 pF, then the actual cut-off frequency fcc is fcc=18 kHz which is lower by 23% from the design value of fcc=23.4 kHz. Namely, the actual resistance and capacitance are displaced from the designed values, then the cut-off frequency is also displaced from the design value.
In ISSCC Digest of Technical Papers, pp. 172-173, Feb. 1996 entitled xe2x80x9cBase and Filters for IS-95 CDMA Receiver Applications Featuring Digital Automatic Tuning,xe2x80x9d it is disclosed that rectangular waves are inputted into the above conventional active filter so that an output signal from the active filter is converted into a digital signal by an A/D converter in order to solve the above problems. This digital signal is operated by a digital signal processor so that in accordance with the operated digital signal, the resistance and capacitance are made varied to adjust the cut-off frequency. This technique is disadvantage in enlarged circuit scale due to provisions of the A/D converter and the digital signal processor.
In the above circumstances, it had been required to develop a novel active filter capable of controlling, resistance and capacitance to adjust a cut-off frequency to suppress variation in the cut-off frequency from a predetermined design value without, however, enlargement of circuit scale.
Accordingly, it is an object of the present invention to provide a novel active filter free from the above problems
It is a flirter object of the present invention to provide a novel active filter capable of controlling resistance and capacitance to adjust a cut-off frequency to suppress variation in the cut-off frequency from a predetermined design value without, however, enlargement of circuit scale.
The present invention provides an active filter having at least an operational amplifier, a plurality of resistances and a plurality of capacitances, wherein at least one of the resistances and the capacitances is variable by controlling a controller connected to an output side of the active filter for receiving an output signal so that the controller counts a frequency of the output-signal to generate a counted value so as to control the at least variable one of the resistances and the capacitances on the basis of the counted value.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.